If you're not gonna ADD any Fuel Filtration capability...
I'd do the Secondary every 10K miles.
The Factory Secondary MAY filter down to 10 Microns NOMINAL.

(I'm using a Racor 3203 as my Secondary, which filters down to 2 Microns.)


According to the FSM the Factory Mercedes Air Filter (For the engine) can be
GENTLY, REVERSE "Blown Out" with air pressure.
[NOT 150 PSI !!!] (I'd not try anything over 1 Bar/14.999999 PSI)

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'84 300SD sold
124.128